Polypropylene films such as biaxially-stretched polypropylene films are used in package materials. To improve heat seal strength, in particular, low-temperature heat seal strength of the polypropylene films, sealant compositions which are blends of resins such as polypropylenes with butene polymers are conventionally known.
For example, JP-A-H11-221884 (Patent Literature 1) discloses a resin composition for sealant film including a polypropylene resin, an ethylene.α-olefin random copolymer and a butene.α-olefin random copolymer.
Further, JP-A-H11-245355 (Patent Literature 2) discloses a sealant composition which is made by blending a specific propylene.1-butene random copolymer with a specific 1-butene.α-olefin random copolymer.
Although films (packages) formed with these compositions show some improvements in low-temperature heat sealability, it is often the case that the films are sticky. Further, difficulties are encountered in controlling the heat seal strength within a certain definite range when heat sealing takes place at low temperatures. In the packaging applications, this causes variations in the strength required to open the packages. Further, the strength required to open packages is apt to change with the lapse of time, and quality control is frequently difficult.